But how effective is this clay shell? Using neutrons, scientists have been able to study how molecules behave when they are trapped by clay particles. Their results suggest that some hazardous chemicals are more mobile than we might have assumed.
Phenol and its derivatives are common chemical compounds, found in many everyday products, from weed-killer to antiseptic solution. Unfortunately phenols are also toxic and can be a serious form of pollution if they reach a water system. It is therefore important to understand how effective clay particles are at mopping up and containing phenol contamination. Using neutron scattering, a Natural Environment Research Council funded team of scientists from University College London, ISIS and Chalmers University in Sweden have been able to watch phenol’s every move.
One of the major advantages of neutron scattering is that it enables scientists to study the molecules under realistic conditions. Unlike other methods of detecting organic particles, neutron scattering still works when the clays are in their natural wet state. “We can see the behaviour at high pressures and temperatures, mimicking the effect of burial down to around 10 km,” explains Neal Skipper, a member of the team. What is more, neutrons can detect phenols at very low concentrations.
To their surprise, neutrons revealed that phenol remains relatively mobile, even inside clays with tiny, nanometresized, pores. Trapping the phenol required minuscule pores, just one molecular layer thick, but even then there is a chance for escape. “If the clay dries and shrinks then cracks are likely to appear, letting the phenol out,” says Skipper. However, it isn’t all bad news. Research by other scientists has shown that clays can be coated with organic cations, which encourage phenols to stick. Skipper and his colleagues now plan to study the ability of these organoclays to immobilise wastes.
And pollution isn’t the only application of this work. Neutron diffraction has also been used to study how other chemical compounds, such as glycol, can be used to stabilise oil and gas wells. They showed that the glycol gets inside clay pores, blocking up the holes and preventing the walls of a well from collapsing.
ISIS neutrons have enabled molecular motions that occur inside some of the most inaccessible places on Earth’s crust to be studied. With work like this we can be confident that hazardous waste stays locked up and doesn’t present a problem for future generations.
Neal Skipper (University College London), n.skipper@ucl.ac.uk
Peter Lock (University College London), Jan Swenson (Chalmers University), Felix Fernandez-Alonso (ISIS)
Research date: December 2006
Further Information
The structure and dynamics of 2-dimensional fluids in swelling clays, NT Skipper et al., Chemical Geology, 230(3) (2006) 182
